The present invention relates to pulse dye lasers. The present invention utilizes a right angle prism to direct radiation from a pump laser onto an optically amplifying medium from two directions, improving the uniformity and maintaining a high efficiency of excitation compared with other means.
A major problem in laser pumped pulse dye lasers has been that of uniform pumping of the dye medium without damage to the system. The most frequently used method for high energy pumping is that of side pumping a dye cell. A positive cylindrical lens is used to focus the pump beam tightly into the dye cell. While this is an efficient means of exciting the active, dye medium, it results in a conical shaped region of excitation that degrades the basic spatial mode of the dye laser.
A four sided pumping scheme that attempts to produce a more uniform excitation was developed by D. Bethune U.S. Pat. No. 4,380,076. The cell for four sided pumping consists of a right angle prism with a bore cylinder through which laser dye flows through the prism body. The pump beam is made four times that of the bore diameter and approximately collimated in the plane orthogonal to the bore cylinder. One quarter of the incident beam pumps the dye in the bore directly without reflection, while the remainder of the beam utilizes one or two total internal reflections to pump the dye from the three remaining directions. Although this process excites the dye medium more uniformly than in the cylindrical lens method, it is not efficient when the pulsed dye laser is linearly polarized and of short pulse duration. In such a four sided pumping scheme, where the direction of polarization of the pump beam is orthogonal to the bore axis, the portions of the pump beam that have experienced an odd number of reflections in the prism are polarized in directions orthogonal to the other beams at the bore cylinder. Consequently, both sets of beams cannot excite the dye molecules uniformly in a single polarization state, matching the polarization of the linearly polarized light that is being amplified. In short pulse systems, the molecules do not have time to reorient in the solvent so a linearly polarized dye beam cannot be amplified efficiently by molecules pumped by radiation in an orthogonal polarization. In the case where the pump radiation is polarized parallel to the bore axis, the number of reflections have no effect on the polarization direction. Unfortunately, dye molecules that have been excited with this polarization and retain their orientation cannot contribute to amplification of the dye laser, regardless of the dye laser polarization, since longitudinally polarized light cannot propagate. Only if the dye laser pulse were to last for a period of time greater than the rotational correlation time of the dye molecules and the fluorescence lifetime of the dye exceed the rotation correlation time could these pumped molecules reorient significantly to contribute to the amplification process. The most efficient optical pumping is achieved when the polarization of the pump beam and the polarization of the dye laser beams are parallel. It should be noted that most pulsed dye lasers utilize a diffraction grating or prisms for frequency tuning and thus inherently produce a linearly polarized laser beam.
Measurements have shown that a pulsed dye amplifier using DCM dye with methanol/propylene carbonate as the solvent, pumped by a 532 nm laser beam with a pulse duration of approximately 10 nanoseconds and polarized orthogonal to the dye laser polarization, was only 20% as efficient as when the polarization of the pump beam and the dye laser were matched. In addition, the orthogonal polarization directions of the pump and dye laser beams gave rise to significant amplified spontaneous emission (ASE) in the orthogonal polarization producing elliptical polarization in the dye laser beam. In addition, an accelerated degradation of the dye also occurs for a given output energy. Since the useful lifetime of many ultraviolet dyes is only a few hours of lasing, this latter consideration is important as a cost factor and as a toxic waste disposal factor.